On the contrary, that is not always prudent counsel to give one. To do something even though one's skill may be lacking, simply because of misguided will, often leads to mis-spent time. If he has judged that three weeks will be enough to write such a story, he is either planning to make it a very brief and surface telling, or simply does not know enough regarding the ways of writing to make an accurate appraisal of such a project.
Think about this. Firstly, a tale of Chrono Trigger's length, to be told in its entirety, would need to be about 400 pages long; that is how long the Novel Project is, I belief. I suppose it could be greatly shorted with less description and the like, but that betrays the very concept of writing it into a novel form. For argument, let us say 200 pages - and that is decidedly brief. 90,000 words is a whisper in the wind compared to what it would need to be, I think. But anyway. Now figure in the estimated time of 2 - 4 weeks. A median figure of three weeks yields a neccessary ten pages per day: a distinct impossibility for anyone but an absolute master writer... strike that, a writer gifted with an extremely swift mind and fingers. At my absolute swiftest I have written five pages in three hours, and that is putting little effort into the exact wording - and I'm no slow typist, either. Taking that as a figure would mean around six hours of work a day for a first draft. A complete editing job requires as much time again. Essentially, to finish a CT novellization in three weeks would require one to spend nearly every waking moment, including school-time, writing... writing without any interruption. It is, in a word, impossible. I would say something of this length... two years is reasonable.
Beyond this is the fact is the very idea of porting exact lines of speech. Very ill-advised. The game is not written in a manner that would work well for a story. A direct copy simply cannot be done - to my mind. Technically, it can be done, but I would not consider such a thing literature or writing to any degree. Every fanfiction that has attempted such a thing has fallen short - and there have been those. Writing is far more difficult than simply typing out lines and adding in description here. To be good it requires many, many, hours of reading and rereading, subtlety, a keen understanding of form and style... it can be done, to be sure, but needs far more patience than a mere three weeks can provide! That does not even give you a chance to have enough of a break from it to give it a fair edit. If you still wish to do such a thing, prepare for a long task of several years. Trust me on that, at least - I once began a CT fanfiction meant to be a short few weeks in writing, sixteen pages long. It was two years and four hundred pages before I finished. These things have a way of... expanding.